1. Field of Invention
This invention relates to improvements in sensing circuits and techniques, and more particularly to improvements in CMOS photo sensing circuits and techniques.
2. Relevant Background
With advancing technology, digital still cameras (DSCs) are becoming increasingly popular, compared with traditional film cameras, in both industry and consumer imaging areas. Typically, the DSCs use one or more charge-coupled-devices (CCDs) to capture the desired light image. The CCDs used mostly in DSC systems, however, have a dynamic range of only about 60 to 70 dB. For most applications this dynamic range is insufficient, since light intensity variation, for instance, in most outdoor photographic scenes has a dynamic range in the order of 120 dB. This suggests a need for an image sensor with some kind of expanded dynamic range.
Many solutions to this problem have been suggested, including logarithmic sensors, multiple frame capture techniques and voltage-to-frequency conversion techniques. Logarithmic sensors incorporate compression at the photodiode level to achieve high dynamic ranges. However, these techniques suffer from the problem of high fixed-pattern noise being generated because of device mismatches. The voltage-to-frequency conversion techniques result in a complex pixel which has a large pixel size and low fill factor. It also suffers under low light conditions. The multiple frame capture techniques require a DSC system capturing multiple frames and use complex post-image processing to reconstruct a wide dynamic range image.
A typical photodiode sensor architecture 10 is shown in FIG. 1, to which reference is first made. The parasitic photodiode capacitance (not shown) is initially charged by activating the reset transistor 12 to connect the photodiode 14 to a voltage source Vreset. When light impinges on the photodiode, charge carriers are generated. The resulting current discharges the photodiode node 16 and decreases the voltage on the photodiode as a function of time.
The relation between the voltage on the photodiode and the photo current I is given by:                               I          =                      C            ⁢                                          ⅆ                V                                            ⅆ                t                                                    ⁢                  
                ⁢                              Δ            ⁢                          xe2x80x83                        ⁢            V                    =                                    I              C                        *            Δ            ⁢                          xe2x80x83                        ⁢            t                                              (        1        )            
where a xcex94V is the photodiode voltage variation from the Vreset level for a given integration time (or exposure time) xcex94t, and Cis the capacitance seen at the photodiode node 16.
For a given integration time xcex94t and constant C, equation (1) shows a linear relationship between xcex94V and I. Once xcex94V is measured at the end of the integration time, the photocurrent I (or alternatively the light intensity) can be calculated based on an I (or light intensity) and xcex94V transfer function shown in FIG. 2, in which the transfer curve 18 is linear.
Due to this simple linear transfer curve 18, most photodiode sensor systems integrate the charge for a particular period of time (or exposure time) and find the light intensity by measuring the photodiode voltage variation. But the fundamental problem behind these type schemes is in the integration time (or exposure time) selection. The photodiode voltage of a low intensity pixel decreases at a very slow rate, and therefore requires a large integration time to be detected with sufficient accuracy. On the other hand, a brightly illuminated pixel has a much higher voltage drop rate, and therefore requires the use of a short integration time to for detection before the photodiode enters saturation, at which time the measured voltage becomes constant. Hence, the light intensity dynamic range that could be detected by using a fixed integration time is limited.
One apparatus that has been proposed to improve the dynamic range for a semiconductor based image sensor generates a photodetector counter value for each pixel of the sensor. Each time the input of a comparator reaches a predetermined threshold value, a counter is incremented and the photodetector reset. At the end of the integration time for a frame capture, the counter value is read out via a digital output bus. The counter value therefore represents the number of times that the photodetector filled to the threshold level and was reset. In addition, the analog voltage of the photodetector is determined at the end of the integration time for a frame capture to represent the amount of charge collected since the photodetector was last reset. The total output value of the pixel at the end of the integration time for a frame capture is then the number of times the photodetector reached the programmed threshold value plus the analog voltage in the photodetector.
The method and apparatus of the present invention use a time domain quantization sensing (TDQS) system that digitizes a pixel analog value by quantizing it in the time domain. As can be seen from the discussion below, the system is implemented in a CMOS sensor system fabricated in a standard CMOS process, instead of a CCD process. A dynamic range of over 130 dB can be achieved in a single frame capture. Other advantages of the sensor include easily programmable resolution, digital FPN calibration, and low readout noise.
According to a broad aspect of the invention, a photodiode sensor is presented. The sensor includes means for providing a photodiode function, having an associated capacitance. The associated capacitance may be, for example, a parasitic capacitance of the means for providing a photodiode function. Means are provided for charging the associated capacitance to a predetermined reset voltage such that when light impinges upon the means for providing a photodiode function, the reset voltage discharges as a known function of time relatable to an intensity of the light. Means are also provided for measuring a time for the reset voltage to discharge to a predetermined threshold voltage to provide an indication of the intensity of the light. Preferably, the means for measuring a time for the reset voltage to discharge to a predetermined threshold voltage is a means for comparing a voltage on the associated capacitance to the predetermined threshold voltage, which may be time varying. The means for measuring a time for the reset voltage to discharge to a predetermined threshold voltage may be a means for sampling an output of the means for comparing a voltage on the associated capacitance at plurality of successive time periods, which also may be time varying.
According to another broad aspect of the invention, a photodiode sensor array is presented. The sensor array includes a plurality of photodiode sensors arranged in a predetermined physical array. Each photodiode sensor has a photodiode having a parasitic capacitance and means for charging the parasitic capacitance to a predetermined reset voltage, wherein when light impinges upon the photodiode, the parasitic capacitance discharges in proportion to an intensity of the light. Each sensor also has means for measuring a time for the reset voltage to discharge to a predetermined threshold value, wherein the time for the reset voltage to discharge to the predetermined threshold voltage indicates the intensity of the light.
According to another broad aspect of the inventions, a photodiode sensor array is presented, which includes a plurality of photodiodes arranged in a predetermined pattern, each having a respective associated capacitance. A circuit is provided for charging the associated capacitances to a predetermined reset voltage such that when light impinges upon the photodiodes, the associated capacitances each discharge in a time proportional to an intensity of the light respectively impingent thereupon. A circuit is also provided for measuring respective times for each of the associated capacitances to discharge to a predetermined threshold value, wherein relative intensities of the light respectively impingent upon each of the photodiodes can be determined.
According to yet another broad aspect of the invention, a method is presented for operating a photodiode sensor array. The method includes providing a plurality of photodiodes arranged in a predetermined pattern, each having a respective associated capacitance. The method also includes charging the associated capacitances to a predetermined reset voltage, and measuring respective times for each of the associated capacitances to discharge to a predetermined threshold value.